Remember, those two are different:
- 80 W tube = rated power output
- 80 power = percentage of power supply’s maximum output current (roughly proportional to tube power output)
If 80% of the maximum power is 35 to 40 mA, then the power supply will deliver about 50 mA at 100%. That is grossly too much for an 80 W tube.
When you run it at 20% to 30%, the power supply produces a current between 10 and 15 mA. @jkwilborn has a table of tube sizes & currents suggesting that’s over half the normal current for an 80 W tube, so any power settings higher than than 50% would constitute tube abuse. 
The rule of thumb is to set 50% for a manual pulse, then adjust the power supply’s internal trimpot to deliver half the tube’s maximum current rating. Then 100% (or 99%, whatever) cannot drive the tube over its maximum current, but you must adjust all your speed & power cut settings for the new reality.